Aiglos78

You'll need to set each companions tactics in order to have them 'hold position' -- you can determine when they choose to engage when you do not have direct control through these options. You can select the 'character' tab in the radial, or go to your inventory, scroll to the bottom of the left pane -- there is a 'head' icon there that represents this option.

If you hold down the right trigger, it will bring up a radial, and the 'talk to' icon looks like a small notecard at the 10 o'clock position. Selecting that will automatically initiate dialogue. You may also notice that if the companion has quest-specific dialogue or they WANT to talk to you, the icon appears on their character portrait.